1. Field of the Invention
The present invention relates to an optical communication system that employs a optical wavelength selector.
2. Description of the Related Art
WDM (Wavelength-division multiplexing) is a method in which optical signals of WDM channels are multiplexed and transmitted by a single optical waveguide. The result of multiplexing WDM channels is called WDM optical signal. A WDM optical network is an optical communication system in which wavelength-division multiplexing is applied in an optical network.
Through the use of wavelength selection devices such as optical wavelength selectors, a WDM optical network enables dynamic change of the network configuration as well as the selection and reception of a desired WDM channel from WDM optical signal. In other words, WDM optical signal in which a plurality of WDM channels are multiplexed can be transmitted by an optical transmission medium such as optical fiber, and a WDM channel can be received from the WDM optical signal on the receiving side.
Optical band-pass filters such as fiber Fabry-Perot filters, acousto-optic filters, and dielectric interference filters have been used as optical wavelength selectors in the prior art, but these filters have been difficult to use because they require accurate control in analog amounts.
In a fiber Fabry-Perot filter, for example, the resonator length of the etalon must be accurately controlled using, for example, a piezoelectric actuator. An acousto-optic filter requires accurate control of the frequency of acoustic waves applied to waveguide using, for example, a voltage-controlled oscillator.
One example of a optical wavelength selector that overcomes the above-described problems combines a wavelength-division demultiplexer, an optical gate switch, and a wavelength-division multiplexer. In this optical wavelength selector, a desired wavelength can be selected by controlling digital values, i.e., the ON/OFF of optical gate switches.
A first example of the above-described optical wavelength selector of the prior art is next described with reference to FIG. 1. This optical wavelength selector is made up of one wavelength-division demultiplexer 20, four optical gate switches 40-43, and one wavelength-division multiplexer 70. Optical gate switches 40-43 are made up of semiconductor optical amplifiers which pass light when ON and cut off light when OFF.
This optical wavelength selector enables selection of any WDM channel from the WDM optical signal of four WDM channels .lambda..sub.0 -.lambda..sub.3 by turning ON any one of optical gate switches 40-43 and turning OFF the remainder.
As the above-described optical wavelength selector, devices have been proposed that use an arrayed waveguide grating device (or a waveguide grating router) as wavelength-division demultiplexer 20 and wavelength-division multiplexer 70. Examples include the device described in M. Zirngibl et. al, "Digitally Tunable Channel Dropping Filter/Equalizer Based on Wavelength Grating Router and Optical Amplifier Integration" in IEEE (Insitute of Electrical and Electronics Engineers) Photonics Technology Letters, Vol. 6, No. 4, April 1994, p. 513.
However, the above-described optical wavelength selector of the prior art has a drawback, that it outputs a plurality of WDM channels from one output port when selecting a WDM channel group, whereas a WDM channel group may be allotted to each node in a WDM optical network, which is an optical communication system, in order to increase the transmission capacity per node.
A prior-art example of the above-described WDM optical network is next described with reference to FIG. 2. In this WDM optical network, each node is provided with four optical transmitters, four optical wavelength selectors, and four optical receivers. For example, node 1 is provided with optical transmitters 110-113, optical wavelength selectors 140-143, and optical receivers 150-153.
Optical transmitters 110-113 in node 1 transmits WDM channels .lambda..sub.0, .lambda..sub.1, .lambda..sub.2, and .lambda..sub.3. Optical transmitters 210-213 in node 2 transmits WDM channels .lambda..sub.4, .lambda..sub.5, .lambda..sub.6, and .lambda..sub.7. Optical transmitters 310-313 in node 3 transmits WDM channels .lambda..sub.8, .lambda..sub.9, .lambda..sub.10, and .lambda..sub.11. Optical transmitters 410-413 in node 4 transmits WDM channels .lambda..sub.12, .lambda..sub.13, .lambda..sub.14, and .lambda..sub.15.
The WDM channels .lambda..sub.0 -.lambda..sub.15 transmitted from the sixteen optical transmitters 110-113, 210-213, 310-313, and 410-413 pass by way of optical fibers 120-123, 220-223, 320-323, and 420-423, which serve as the optical transmission medium, and are combined by star coupler 100. This combined light is outputted to all of optical fibers 130-133, 230-233, 330-333, and 430-433.
Upon receiving a signal transmitted from, for example, node 1, each node selects and receives WDM channels .lambda..sub.0, .lambda..sub.1, .lambda..sub.2, and .lambda..sub.3, and upon receiving a signal transmitted from node 2, selects and receives WDM channels .lambda..sub.4, .lambda..sub.5, .lambda..sub.6, and .lambda..sub.7. If data are transmitted from node 1 to node 2, WDM channels .lambda..sub.0, .lambda..sub.1, .lambda..sub.2, and .lambda..sub.3 are each selected by optical wavelength selectors 240, 241, 242, and 243 of node 2.
In this WDM optical network, four optical transmitters, four optical wavelength selectors, and four optical receivers are required at every node, and as a result, the cost for each node is approximately four times the cost for a case in which one WDM channel is assigned to each node.
In other words, when using an optical wavelength selector of the prior art in a WDM optical network configured as shown in FIG. 2, there is the problem that the cost per node increases in proportional to the number of WDM channels included in each WDM channel group.
Moreover, in the optical wavelength selector of the first example of the prior art shown in FIG. 1, the number of optical gate switches 40-43 required is equal to the number of WDM channels.
In the above-described example of the prior art, four optical gate switches 40-43 are required because four WDM channels are multiplexed. Similarly, the multiplexing of 32 WDM channels calls for 32 optical gate switches.
Optical gate switches are active elements that consume electrical power to operate, and increasing the number of optical gate switches is therefore disadvantageous due to the accompanying increases in both the scale of the device and power consumption.
An optical wavelength selector developed with the object of solving the above-described problem is disclosed by Y. Tsuchikawa and Y. Inoue in Electronics Letters (Vol. 31, No. 23, November 1995, pp. 2029-2030).
The art disclosed in this paper is next described as the second example of an optical wavelength selector of the prior art with reference to FIG. 3. This optical wavelength selector is made up of one optical splitter 20; four optical gate switches 40-43, one optical wavelength router 60; eight optical gate switches 400-407, and one optical combiner 470.
This optical wavelength selector can select any WDM channel from WDM optical signal in which 32 WDM channels .lambda..sub.0 -.lambda..sub.31 are multiplexed.
In brief, turning ON one of the four optical gate switches 40-43 of the first stage selects eight specific WDM channels from the 32 WDM channels .lambda..sub.0 -.lambda..sub.31. Then, turning ON one of the eight optical gate switch 400-407 of the second stage selects one WDM channel from these eight WDM channel.
The WDM channels which passes from each input ports to each output ports of the above-described optical wavelength router 60 is as shown in Table 1 below.
TABLE 1 OUTPUT INPUT o0 o1 o2 o3 o4 o5 o6 o7 i0 .lambda.0 .lambda.1 .lambda.2 .lambda.3 .lambda.4 .lambda.5 .lambda.6 .lambda.7 i1 .lambda.8 .lambda.9 .lambda.10 .lambda.11 .lambda.12 .lambda.13 .lambda.14 .lambda.15 i2 .lambda.16 .lambda.17 .lambda.18 .lambda.19 .lambda.20 .lambda.21 .lambda.22 .lambda.23 i3 .lambda.24 .lambda.25 .lambda.26 .lambda.27 .lambda.28 .lambda.29 .lambda.30 .lambda.31
The above-described optical wavelength selector enables selection of one WDM channel from 32 WDM channels using 12 optical gate switches 40 by arranging optical gate switches 40 in two stages of four and eight.
In the above-described optical wavelength selector, however, optical splitter 20 carries out 1.times.4 branching, which gives rise to a minimum loss of 6 dB. Optical combiner 470 furthermore carries out 8.times.1 combining, which gives rise to an additional minimum loss of 9 dB.
As a result, the intensity of the one optical signal outputted by the above-described optical wavelength selector tends to be inadequate. If the above-described optical wavelength selector is used in a WDM optical network, the received intensity may be inadequate as described above and therefore cause errors in communication.
In addition, although the above-described optical wavelength selector enables a reduction in the number of optical gate switches 40, further reductions are hoped for.